An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into multiple different products. Based on the processing methods, oil products can be separated in different classes, such as crude fractions of petroleum products, light refined oil, and heavy residual oil. Crude fractions of petroleum products include natural gasoline, diesel, kerosene, asphalt, vacuum oils, naphtha, coke, and other petroleum products. Refined petroleum products include regular and reformulated gasoline, jet and fuel kerosene, lube oils, number 2 fuel oil, number 6 fuel oil, and other petroleum products. Each type of petroleum product is characterized by a particular type of hydrocarbon molecules. The hydrocarbon molecules of petroleum products can have varying lengths and complexity. The differences in the structure of the hydrocarbon molecules account for their varying physical and chemical properties, making each type of petroleum product useful for particular applications.
Petroleum products are susceptible to contamination. Analytical testing for contaminants in a petroleum fluid is an important step in many industrial processes. Contaminants can be in the form of small amounts of other types of petroleum products, which have remained, for example, in multi-purpose pipelines or refined oil storage tanks. Contaminants can be in the form of small amounts of the same petroleum product but having different sulfur contents, which commonly happens in diesel fuel distribution, operations, and storage. The contaminants can also be in the form of weathered petroleum products mixed with fresh petroleum products or in the form of some chemicals that cannot be readily identified. Identification of contamination of petroleum products can be used to discover contaminant sources, to correct contamination or to prevent future contamination.